1. Field of the Invention
The present invention relates to a process for electrochemically metallizing an insulating substrate.
2. Description of the Related Art
Various processes for metallizing insulating substrates, especially for the purpose of manufacturing mirrors by metallizing glass plates, are known.
The oldest processes consist in bringing the insulating plate to be metallized into contact with a solution of a metal salt and of a reducing solution which causes precipitation. The contacting may take place by spraying or by immersion. These processes require the use of a mixture of salts and optionally of additives. Furthermore, they do not make it possible to control either the rate of deposition or the texture, that is to say the quality of the coating obtained.
More recently, processes based on vacuum evaporation deposition have been developed. This technique, very simple in its principle, requires a vacuum to be created before-hand in a chamber in which the evaporation of the metal will take place. The films obtained by vacuum evaporation are in general of good quality, but the high cost of the process limits their use to particular applications such as the production of small mirrors, for example rear-view mirrors used in motor vehicles or mirrors used in optics.
It is known to produce metal coatings on metal substrates electrolytically. Many applications have been developed with very good results. However, it is also known, especially from J. Dini, [Electrodeposition, Noyes Publication, Park Ridge N.J., USA (1992), page 195], that the use of the process with high growth rates causes an irregular dendritic or pulverulent growth. Such coatings are unuseable for industrial applications, as they break down into powder. One solution for limiting or eliminating the formation of dendrites during electrolytic deposition of a metal film on a conducting substrate consists in adding additives to the electrolyte. However, this process is essentially empirical. Good results may be obtained, but they are not easily reproducible. Furthermore, a slight change in the additive content or in its nature may cause considerable changes to the film deposited.
Independently, trials have been conducted in order to transpose the process of electrolytic deposition onto a metal substrate to the metallization of insulating substrates, for example the metallization of glass plates. For example, V. Fleury [xe2x80x9cBranched fractal patterns in non-equilibrium electrochemical deposition from oscillatory nucleation and growthxe2x80x9d, Nature, Vol. 390 November 1997, 145-148] describes a process for the galvanic deposition of a copper film on an insulating substrate. That surface of the insulating substrate which is to be metallized is covered with a thin gold film. The substrate is then placed in a solution of a copper salt and is connected to an anode consisting of copper and a cathode consisting of a gold film, the two electrodes being connected to a current generator. The coating on the surface of the insulating substrate is obtained by the reduction of copper at the cathode. The reduced metal starts by being deposited at the cathode, and then the deposition continues on that surface to be metallized which is covered with the thin non-conducting film of gold. However, this case also results in a dendritic growth which does not form a completely covering uniform thin film. On the contrary, the structure of the coating is extremely arborescent and tortuous. It is known that, in such arborescent growth, the choice of current density imposed on the electrochemical cell allows the rate of formation of the metal coating to be modified, an increase in the current density producing an increase in the deposition rate. However, it has been found that an increase in the current density causes the formation of pulverulent dendritic coatings. Thus, T. R. Bergstrasser and H. D. Merchant [xe2x80x9cSurface Morphology of Electrodepositsxe2x80x9d, pp. 115-168 in Defect Structure, Morphology and Properties of Deposits, Proceedings of the Materials Week Rosemont 1994, Publication of the Minerals-Metals-Materials Society, ed. by H. D. Merchant] show that the higher the current intensity used with respect to the equilibrium current intensity, the more the coatings formed are pulverulent in nature. The three-dimensional powders thus obtained are of no industrial use, their sole advantage being to allow the fundamental study of fractal dendritic growth (J. Dini, ibid. page 175).
The research carried out by the inventor has allowed him to show that, when an electrochemical process is used to grow powders along the surface of a substrate, by applying a markedly higher current density to the electrochemical cell than the current densities above which, according to the prior art, only three-dimensional powders could be obtained, a coating on the substrate was obtained in which the grains are arranged so as to form a uniform covering film and no longer dendrites.
This is why the subject of the present invention is an electrochemical process for the deposition of a continuous thin metal film on an insulating substrate.
The process for metallizing an insulating substrate by depositing a uniform thin film of a metal M on the said insulating substrate consists in placing the said substrate in an electrochemical cell which contains as electrolyte a solution of a salt of the metal M in a solvent and which comprises an anode consisting of the metal M and a cathode in direct contact with the said insulating substrate, then in carrying out an electrolysis at constant current, the said process being characterized in that:
a conducting film, which will constitute the cathode, is initially applied to one end of the substrate;
the substrate is placed in the electrochemical cell in such a way that the surface to be metallized is vertical and the cathode is located in the upper part; and
a current is imposed on the electrochemical cell with an intensity such that it creates a current density of between 1 and 50 mA/cm2 in the horizontal section of the electrochemical cell level with the growth front of the film which is deposited.